Infection of rhesus macaques with SIV has proven to be a valuable model for the testing of new therapeutic strategies (including vaccines) for the treatment of HIV-1 infection and AIDS. In this proposal we plan to investigate several important hypotheses as they relate to growing evidence that the HIV-1 co-receptor CCR5 exists in multiple conformational states on CD4+ T-cells, that these conformational states may effect the permissiveness of HIV-1 entry and that blocking CCR5 with high-affinity human anti-hCCR5 Mabs may be a cost effective and viable strategy to prevent the vertical transmission of HIV-l in certain clinical settings. We have chosen the well characterized macaque model of SIVmac239 infection for these studies. Specifically, we will use our 15 billion member human single chain antibody (sFv) phage display library to isolate high affinity human anti-RhCCR5 antibodies against different conformational states of rhesus macaque CCR5 (RhCCR5). We will perform epitope mapping studies and determine the sFv binding constants for the anti-RhCCR5 sFvs. We will use multiparameter FACS to investigate the existence of multiple conformational states of RhCCR5 on different subpopulations of macaque CD4+ T-cells by examining the anti-RhCCR5 sFv binding profiles under resting and different conditions of stimulation. We will also investigate whether certain conformational states of RhCCR5 may be more permissive for SIV entry by testing the anti-RhCCR5 sFvs both individually and in combinations for their ability to block in vitro SIV infection of macaque CD4+ T-cells. We will also determine if co-receptor adaptation by SIV occurs in vitro as a result of antibody treatment. We will also perform "in vitro" affinity maturation on the most promising SIV neutralizing anti-RhCCR5 sFv to test the hypothesis that increased anti-viral activity can be achieved by improving sFv binding affinity to the nM or sub-nanomolar range. Finally, we will use these high-affinity and SIV neutralizing anti-RhCCR5 sFvs to prepare chimeric macaque IgG1 Mabs to determine if whole anti-RhCCR5 Mabs have enhanced neutralizing activity in SIV challenge studies of macaque CD4+ T-cells. These macaque anti-RhCCR5 Mabs will be used in in vivo studies to test the hypothesis that passive immunotherapy with macaque anti-RhCCR5 Mabs can be used to block the vertical transmission of SIVmac239. These studies will supply important information that may be directly applicable to similar human clinical settings.